Various substances and compositions have been used to establish electrical linkages with mammaliam (human) skin, but all those developed to date suffer from a number of disadvantages. Saline solutions are not neat in use and are prone to evaporate, thereby changing their concentration and leading to the introduction of errors and complication in electrical base line shifts. Furthermore, they are difficult to use as they must be kept in place by some other device, for instance a suction bulb which can result in raising welts on the skin of the patient. Conductive gels and creams have the advantage that they do not readily evaporate, but they are also far from neat to use and must be cleaned up after completion of measurements not only from the patient but from the equipment as well. Also the salt content of the adhering residue of such gels and creams cause the metal parts of the measuring equipment in contact with the residues to tarnish, oxidize, and ultimately fail. Moreover, since these creams and gels are rather fluid, as the mass of the gel moves or flows, the electronic monitoring equipment interprets this as electrical action and therefore reports it as such, to cause a defect known as motion artifact which is a nuisance in most cases and sometimes even leads to false findings. Pads or semi-solid gels have been proposed to overcome these difficulties. For example U.S. Pat. No. 3,998,215 describes a gel pad which consists of an electronically conductive hydrogel and an electrode of a sensing device impregnated in a porous matrix or held within a cavity. This invention relies on a fibrous stiff carrier. However, this device suffers from a number of disadvantages. The pads tend to lose water through evaporation and have lower contactivity and higher impedance than do the creams and gels. Another attempt to resolve these problems if found in U.S. Pat. No. 4,125,110. Based on Karaya gum which is somewhat difficult to handle once wetted, as it tends to cake, the composition contains less than 3% water, nominally, and almost no salt, that is neither sodium chloride nor calcium chloride. Thus, its conductivity and ability to pass electrical signals and pulses is greatly impaired and it is less effective for the use intended.
Other U.S. patents of potential interest in connection with this invention are U.S. Pat. Nos. 3,993,049; 4,066,078; 4,215,696; 4,105,033; 3,743,613; 3,734,820; 3,833,518; 4,097,430; 3,301,723; 3,249,109; and 4,274,420.
According to the present invention, these problems are solved without a fiberous carrier and without having to resort either to low water and low salt content or to messy gels and creams which yield unavoidable residues.